Remove #find_object override in CI job mutations

What does this MR do and why?

Remove #find_object override in CI job mutations

This override does not gracefully handle a RecordNotFound exception, which causes a 500 response when a non-existing ID is provided.

References

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MR acceptance checklist

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Screenshots or screen recordings

Response before:

{
  "errors": [
    {
      "name": "ServerError",
      "response": {},
      "statusCode": 500,
      "result": {
        "errors": [
          {
            "message": "Internal server error: Couldn't find Ci::Build with 'id'=-1 [WHERE \"p_ci_builds\".\"type\" = $1]",
            "raisedAt": "..."
          }
        ]
      },
      "message": "Response not successful: Received status code 500",
      "stack": "ServerError: Response not successful: Received status code 500\n    at throwServerError ..."
    }
  ]
}

Response after:

{
  "errors": [
    {
      "graphQLErrors": [
        {
          "message": "The resource that you are attempting to access does not exist or you don't have permission to perform this action",
          "locations": [
            {
              "line": 2,
              "column": 3
            }
          ],
          "path": [
            "jobRetry"
          ]
        }
      ],
      "clientErrors": [],
      "networkError": null,
      "message": "The resource that you are attempting to access does not exist or you don't have permission to perform this action",
      "stack": "Error\n    at new ApolloError2 ..."
    }
  ]
}

How to set up and validate locally

Visit https://gdk.test:3443/-/graphql-explorer and execute the following query:

mutation jobRetry {
  jobRetry(input: {id:"gid://gitlab/Ci::Build/-1"}) {
    errors
  }
}